Effective separation of highly hydrophilic anions (e.g., sulfate, selenate, chromate, phosphate) from competitive aqueous solutions remains a major challenge, despite the tremendous progress in anion receptor chemistry over the past decade. In the particular case of sulfate, although a significant number of sulfate-binding receptors have been reported (e.g., I. Ravikumar et al., Chem. Soc. Rev., 41, 3077, 2012), they have shown limited success in the substantial removal of this anion from water. A significant obstacle in the development of anion receptors is often the multi-step synthesis required for their assembly, which generally involves tedious purifications and the use of toxic reagents and solvents.
The removal of sulfate from seawater, in particular, continues to be an ongoing challenge. Seawater contains very high levels of sulfate (˜3,000 mg/L), and seawater is used on a large scale in oil-field injection operations. During such operations, the sulfate in the seawater combines with strontium and barium found in rock to form barium and strontium sulfate scale. The precipitation of barium and strontium sulfates is highly detrimental to the process, such as by clogging lines and destroying production wells. The conventional technology for removing sulfate from seawater is by nanofiltration, which can reduce sulfate levels to about 50 mg/mL. However, some drawbacks to this approach are the remaining high sulfate levels, the need to pressurize the system to 20-30 bars, which results in a significant expenditure in energy, and membrane fouling. Other methods involve scale-removing chemicals, but these are known to be difficult to use and very expensive, and are not very effective against sulfate scales. Another technology, known as the MD-LPP process, yields sulfate-free seawater, but the process has the significant drawbacks of employing high pressures, pre-concentrating the sea water, and use of organic solvents.
An approach for aqueous anion separation that has proven particularly effective is selective anion crystallization with organic compounds functionalized with hydrogen bonding groups (e.g., a) R. Custelcean, Curr. Opin. Solid State Mater. Sci. 2009, 13, 68; b) R. Custelcean, Chem. Soc. Rev. 2010, 39, 3675; c) R. Custelcean, Chem. Commun. 2013, 49, 2173). This approach combines elements of anion receptor chemistry and crystal engineering, as it entails recognition of the targeted anion through complementary hydrogen bonding, and formation of stable crystals through favorable packing. The challenge with anion crystallization from water is to identify anion-binding compounds that can effectively compete against the strong anion hydration, and that are also able to self-assemble with the anions of interest into crystals with low aqueous solubility. In this respect, it has recently been discovered that crystallization of sulfate, in the form of extended [SO4(H2O)52−]n clusters, with rigid and planar bis-guanidinium compounds, can strike a favorable energetic balance that allows for the efficient separation of the highly hydrophilic sulfate anion from water (R. Custelcean et al., Angew. Chem. Int. Ed. 54, 10525, 2015; Angew. Chem. 127, 10671, 2015). In the foregoing prototype, the bis-guanidinium compound was synthesized in situ by condensation of glyoxal with aminoguanidinium sulfate, resulting in a sulfate salt with low aqueous solubility (Ksp=3.2×10−7), comparable with that of SrSO4. Although the solubility of the foregoing bis-guanidinium sulfate salt is much lower than many other organic sulfate salts, the solubility remains unacceptably high, particularly for use in oil-field injection operations involving competitive aqueous solutions of high ionic strength, such as seawater. There would be a significant benefit in a straight-forward and cost-efficient process that could remove substantially all sulfate from seawater without the use of pressure, nanofiltration, pre-concentration, and organic solvents.